The present invention relates to a novel group of cladding designs, especially for use in optical fibres, wherein a larger photonic bandgap may be obtained to confine light in hollow cores.
Optical fibres and integrated optical waveguides are today applied in a wide range of applications within areas such as optical communications, sensor technology, spectroscopy, and medicine. These waveguides normally operate by guiding the electromagnetic field (the light or the photons) through a physical effect, which is known as total internal reflection. By using this fundamental effect, the propagation (or loss) of optical power in directions perpendicular to the waveguide axis is reduced.
In order to obtain total internal reflection in these waveguides, which are often fabricated from dielectric materials (in optical fibres) or semiconductors (in integrated optics), it is necessary to use a higher refractive index of the core compared to the refractive index of the surrounding cladding.
Today the preferred signal transmission medium over long and medium distances is the optical fibre, and total internal reflection is, consequently, a physical property, which has been known and used in technological development for decades. During the past ten years, however, the development within the area of new materials has opened up the possibilities of localisation of light or control of electromagnetic fields in cavities or waveguides by applying a completely new physical propertyxe2x80x94the so-called photonic bandgap (PBG) effect.
The PBG effect may be introduced by providing a spatially periodic lattice structure, in which the lattice dimensions and applied materials are chosen in such a way that electromagnetic field propagation is inhibited in certain frequency intervals and in certain directions. These PBG materials have been described in one-, two-, and three-dimensional cases in the scientific literature and in several patents (see for instance U.S. Pat. Nos. 5,386,215, 5,335,240, 5,440,421, 5,600,483, 5,172,267, 5,559,825).
A specific class of components, which makes use of such periodic dielectric structures, are the optical fibres (or waveguides), in which the periodic variation appears in directions perpendicular to the waveguide axes, whereas the structures are invariant along the waveguide axes.
Within recent years, especially researchers from University of Bath, UK, (see e.g. Birks et al., Electronics Letters, Vol.31 (22), p. 1941, October 1995) have presented optical fibres realised by having a core surrounded by thin, parallel, and air-filled voids/holes in a silica-background material, and organising the air-filled voids in a periodic structure in the cladding region of the fibres.
Although the above-cited Birks et al reference discloses the idea of photonic bandgap guiding fibers, it has since then been realised that the requirement that the cladding structure exhibits photonic bandgap effect is not necessary for these so-called microstructured fibers to be able to guide light (see e.g. Knight et al., Journal of the Optical Society of America, A., Vol.15 (3), p.748, March 1998). The reason for this is that microstructured fibers, which have a core region with a higher refractive index than the effective refractive index of the cladding structure, are able to guide light by total internal reflection. In accordance with this, it has also been realised that a periodic arrangement of the air voids is not a requirement for the operation of high-index core microstructured fibers (see e.g. U.S. Pat. No. 5,802,236).
It is important to notice that all of the high-index core microstructured fibers, which have been demonstrated, have not had an operation based on photonic bandgap effects. But simply due to the higher refractive index of the core region compared to the cladding (see e.g. U.S. Pat. No. 5,802,236 for definition of the core and cladding indices), all high-index core fibers have a fundamental mode which is guided due to total internal reflection (also known as index guiding).
In contrast to the high-index core fibers, low-index core fibers (i.e. fibres having a core region with a lower refractive index than the cladding) are not able to guide light leakage-free in the core region through total internal reflection. However, by designing a periodic cladding structure correctly, this cladding structure is able to exhibit photonic bandgap effects, as described in the above-cited Birks et al. reference.
Designing the cladding structure correctly involves optimising the periodic arrangement of voids with respect to sizes, dimensions, and morphology. Cladding structure which are exhibiting photonic bandgap effects are able to reflect light of certain wavelength and incident angles. This means that the cladding structure is able to confine light, which satisfies the condition that the light falls within a photonic bandgap, to a spatial region surrounded by the cladding structure. This is even the case when the spatial region has effectively a lower refractive index than the cladding structure. This is the operational principle of PBG guiding optical fibres and other PBG waveguides (see e.g. Barkou et al., Optics Letters, Vol.24 (1), p. 46, January 1999).
Due to the radically different physical mechanism causing the waveguidance, microstructured fibers classify into (at least) two groups. Namely those that are operating by photonic bandgap effect, which we will call PBG fibres (we will also refer to them as bandgap fibres or low-index core fibers), and those operating by total internal reflection, which we will refer to as high-index core fibres or index-guiding fibres.
Waveguidance by photonic bandgap effects are of significant future interest, as it allows radically new designs of optical fibres and other types waveguides. In particular for optical fibres, the core is not required to have a higher refractive index than the cladding. Such low-index core optical fibers (e.g. hollow core fibers) may be exploited in numerous applications, e.g. in sensor systems or for use as an ultra-low loss transmission fibre in telecommunication systems.
Recently the first photonic bandgap guiding optical fibre was demonstrated (see Knight et al., Science, Vol.282 (5393), p. 1476, November 1998). The design of this fibre was based on a Honeycomb arrangement of air voids in a silica background material in the cladding, and a single periodicity-breaking low-index region formed the core. The advantages of using a Honeycomb-based cladding structure compared to e.g. a triangular structure are that the cladding structure exhibits photonic bandgap effects for smaller (and thereby more realistic) air void sizes.
It is a disadvantage that, due to the triangular cladding structure, the PBG of the structures described by Birks et al. are not optimised for guiding electromagnetic radiation using the PBG effect.
It is a further disadvantage that the light in the recently demonstrated Honeycomb based PBG fibre is distributed almost entirely in silica.
It is a still further disadvantage that the cladding structure in the recently demonstrated Honeycomb-based PBG fibre is not optimised for guiding light inside a hollow core.
It is a still further disadvantage that the honeycomb arrangement of air voids in the cladding structure in the recently demonstrated PBG fibre is not optimised for obtaining a large void filling fraction.
It may be a problem or disadvantage of the present realisation of optical fibres with periodic dielectric cladding regions that careful, close-packed stacking of either hexagonal rods and hexagonal glass tubes (with central voids) or direct stacking of circular rods and thin circular tubes is required. These tubes and rods have been arranged in a close-packed triangular structure in a preform, where after the preform has been drawn into an optical fibre. Although these fibres according to the reports in the international literature show quite interesting and new optical properties, one of the disadvantages has been that the close-packing of the tubes and rods is not optimised for realising fibres with large void filling fractions.
As known to those skilled in the art large void filling fractions are required for obtaining PBG fibres where light is guided substantially inside a hollow core.
Thus, it is a further disadvantage that the present stacking of either hexagonal glass tubes (with central voids) or direct stacking of thin circular tubes in a close-packed structure is not optimised for fabricating optical fibres with large void filling fractions. U.S. Pat. No. 5,802,236 discloses micro-fabricated optical fibres having a core and a cladding region, wherein the cladding region comprises a multiplicity of spaced apart cladding features that are elongated in the direction of the fibre. The effective refractive index of the cladding region is less than the effective refractive index of the core region. Furthermore, the elongated features in the cladding are arranged in a non-periodic structure.
It is a disadvantage of the micro-fabricated optical fibre disclosed in U.S. Pat. No. 5,802,236 that due to the high-index core region the waveguiding characteristics are based on traditional total internal reflection of the electromagnetic radiation guided in the core region.
It is a further disadvantage of the micro-fabricated optical fibre disclosed in U.S. Pat. No. 5,802,236 that the non-periodic cladding structure will not be able to exhibit photonic bandgap effects. The non-periodic fibres disclosed in U.S. Pat. No. 5,802,236 will, therefore, only be able to guide light by traditional total internal reflection.
It is a further disadvantage for all high-index core fibres that these fibres will always support a fundamental mode which is guided due to total internal reflection. This naturally has the consequence that for applications within areas such as optical sensors and low-loss transmission links in telecommunications, where it may be of specific interest to be able to localise optical fields in a single, well-known mode-distribution within areas of low refractive indices (e.g. in vacuum, liquid- or gas-filled channels), the presently known high-index core fibres may not be used directly.
WO 99/00685 discloses a large core photonic crystal fibre (PCF) comprising a cladding having preferably a triangular periodic structure. The core region may be either a high-index or low-index region having a diameter of at least 5 xcexcm. In a preferred embodiment, the fibre is guiding by total internal reflection, and has a solid core region made from pure, undoped silica and may be as large as 50 xcexcm in diameter. With such a diameter, the fibre is capable of transmitting high powers, whilst maintaining sinlge-mode operation for sufficiently small air voids (see e.g. Knight et al., Electronics Letters, Vol.34 (13), p. 1347, June 1998). The reason for the single-mode operation is that the fibre in the preferred embodiment with a large solid silica core surrounded by a silica material with small air voids has a very little contrast between the effective refractive core index (equal to silica) and the effective cladding index. Thereby higher order modes can be avoided for this fibre configuration. It is again important to notice that the large core fibre with a high-index core is operating by traditional total internal reflection, and is therefore not able to confine light in a hollow core.
It is a disadvantage that the triangular cladding structure disclosed in WO 99/00685 are not optimised to provide a sufficient PBG effect so as to effectively confine visible or near-infrared electromagnetic radiation within a low-index core region of the fibre.
It is a further disadvantage of the structure disclosed in WO 99/00685 that in order to obtain a single-mode operation for the fibre with a large silica core, only very small air voids are allowed in the cladding. Thereby the fibre will have a very low contrast between the coreindex and the effective cladding index, which has the negative consequence that the guided mode(s) will not be strongly confined to the core region. The fibre will, therefore, be very sensitive to both micro- and macro-bends, and will experience losses under normal operation of e.g. fibres for telecommunications. The fibre, disclosed in WO 99/00685, is therefore not optimised for leakage-free transmission of high optical powers in a real environment.
It is an object of the present invention to provide a new class of optical waveguides, in which waveguiding along one or more core regions is obtained through the application of the PBG effect.
It is a further object of the present invention to provide optimised two-dimensional lattice structures capable of providing complete PBGs, which reflects light incident from air or vacuum. Such structures may be used as cladding structures in optical fibre, where light is confined and thereby guided in a hollow core region.
It is a still further object of the present invention to provide designs for ultra low-loss PBG waveguiding structures.
It is a still further object of the present invention to provide PBG structures, which are easy to manufacture.
It is a still further object of the present invention to provide a new fabrication technique, which allows easy manufacturing of photonic crystal fibers with large void filling fractions, as well as it allows a high flexibility in the design of the cladding and core structures.
For utilisation of PBG effects in optical fibres (as well as other types of waveguides and components) it is vital to be able to realise cladding structures which exhibit wide bandgaps as well as bandgaps which extend below the so-called air line. That the bandgaps extend below the air line means that the cladding structure is able to reflect light which is incident from air (or vacuum).
As known to those skilled in the art, the two main factors for obtaining these goals are realisation of structures with large void filling fractions, and proper design of the technique which does not only allow fabrication of fibres with larger void filling fractions than what is presently possible, but also greatly increases the flexibility of designing the morphology of the final fibre. Furthermore the present inventors have realised how to modify the size of the photonic bandgap of an optical fibre, in which the cladding structure is formed as two-dimensionally periodic low-index areas within a given material. If there in such a structure is defined a number of high-index areas, which are separated by the low-index areas forming the periodic structure, then the performance of the optical bandgap may be increased, if either the separation between these high-index areas, their respective refractive indices, or both are increased. The high-index areas couple via xe2x80x9cbridgingxe2x80x9d areas between the low-index areas, and their separation may be obtained in a number of areas.
It is known from the international literature (Broeng et at., Optics Communications, Vol.156 (4-6), p. 240, November 1998) that it may decrease the size of the photonic bandgap, if interstitial voids are introduced in a triangular cladding structure of a photonic crystal fibre. For this reason, it would seem most reasonable to make a fibre design, which would tend to eliminate the interstitial voids if a triangular cladding structure is used.
However, the present inventors have realised that it is important where interstitial voids are placed, and they may indeed be advantageous, if they are located at places different from the immediately natural locations (the locations seen as a result of the prior art fabrication technique). As illustrated in the following description, the key point is to place the interstitial voids in such a manner that they further separate the high-index areas of the periodic cladding structure. The present invention not only includes a number of preferred embodiments for the positioning of the interstitial voids, it also includes a new fabrication technique making manufacturing of microstructured fibres with the desired positioning of the interstitial voids possible.
In the following, naturally a periodic structure will be defined by a primitive unit cell, as is the most widely used manner of simplifying the analysis of such a structure. It should be noted that many sizes of unit cells will exist but only one size of a primitive unit cell which is defined as a unit cell which has the smallest area (or volume for 3D periodic structures) possible and which, only by translation, may generate the structure. Naturally, a given periodic structure may have a plurality of primitive unit cells.
In the following, the structure is defined by a unit cell, which will be identical to a primitive unit cell.
In the present context, xe2x80x9cpositioned substantially along the linexe2x80x9d will mean that it is desired to have elements positioned with centres directly on the connecting line between two adjacent primary elements, but that the manners of production will often alter this. In the prior art it is seen that the position of circular air voids (low-index areas) may be controlled to within 10% of the center-to-center distance between two adjacent primary elements, which is a sign of this substantial positioning.
Also, in the present context, the refractive index of the primary elements has to be lower than that of any material adjacent thereto, meaning that this actual change of index is the one providing the periodic structure. This step is not dependent on changes of refractive indices outside the immediate area around the circumference of the primary elements. Naturally, this step may be different for all primary elements, but usually the material adjacent to the primary elements is the same throughout the structurexe2x80x94and so is that of the primary elements, whereby the step will be the same at all circumferences around the primary elements.
In a first aspect, the present invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:
a core region extending along the longitudinal direction,
a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the elongated elements having a refractive index being lower than a refractive index of any material adjacent to the elongated elements,
xe2x80x83the periodic structure being, in a cross-section perpendicular to the longitudinal direction, defined by at least one unit cell, wherein, for each unit cell:
any distance between centre axes of two neighbouring elongated elements does not exceed 2 xcexcm, and
the sum of all areas of all elements, which areas are comprised within a given unit cell, is larger than 0.15 times the area of that unit cell.
Dependent on the wavelength at which the optical fibre is intended to operate, the distance between centre axes of two neighbouring elongated elements may even become smaller than 1.9 xcexcm, such as smaller than 1.8 xcexcm, such as smaller than 1.6 xcexcm, such as smaller than 1.4 xcexcm, such as smaller than 1.2 xcexcm, such as smaller than 1.0 xcexcm, such as smaller than 0.8 xcexcm, such as smaller than 0.6 xcexcm.
For a given unit cell, the sum of all areas of all elements within the unit cell may preferable be larger than a constant times the area of that unit cell, said constant being larger than 0.2, such as larger than 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5, such as larger than 0.6, such as larger than 0.7, such as larger than 0.8.
For each unit cell a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any elongated elements, and wherein the centres of those elongated elements, parts of which are within a distance of 1.5 or less, such as 1.2 or less, such as 1.1 or less times the radius of the first circle from the centre of the first circle, define the vertices of a polygon with three or more sides. The polygon may be a regular a triangular, rectangular, quadratic, or hexagonal polygon.
The optical fibre according to present invention may further comprising further, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide.
These further, elongated elements have a refractive index being higher than a refractive index of any material adjacent to the secondary, elongated elements, and each having a centre not positioned outside the unit cell, and each having an area not exceeding the area of the unit cell.
Part of the further, elongated elements, in the cross-section, define a triangular structure, or a Honeycomb structure, or a Kagomxc3xa9 structure.
In the present context, a Honeycomb structure is defined as a hexagonal polygon, all sides of which are common to another hexagonal polygon. By a Kagomxc3xa9 structure is meant a structure defined by a hexagonal polygon and a regular triangle having a side length corresponding to that of the hexagonal polygon, and where hexagonal polygons exist, each side of which is common to a triangle.
The further, elongated elements, in the cross-section, are at least partly comprised within the first circle. Preferably, the centres of at least part of the further, elongated elements substantially coincide with the centre of the first circle.
In a second aspect, the present invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:
a core region extending along the longitudinal direction,
a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising:
primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the primary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
secondary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the secondary elements having a refractive index being lower than a refractive index of any material adjacent to the secondary elements,
xe2x80x83wherein any area of any primary element is larger than any area of any secondary element, and wherein
xe2x80x83the periodic structure being, in a cross-section perpendicular to the longitudinal direction, defined by at least one unit cell, wherein, for each unit cell:
the sum of the areas of secondary elements, which areas are comprised within a given unit cell, is larger than 0.09 times the area of that unit cell.
Any area of any primary element is larger than a constant times any area of any secondary element, said constant being larger than 1.1, such as larger than 1.2, such as larger than 1.3, such as larger than 1.4, such as larger than 1.5, such as larger than 2, such as larger than 5, such as larger than 10, such as larger than 15, such as larger than 20, such as larger than 50.
To provide a large air filling factor, the sum of all areas of the secondary elements within the unit cell is larger than 0.1, such as larger than 0.15, such as larger than 0.2, such as larger than 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5, such as larger than 0.6.
For each unit cell a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any primary, elongated elements, and wherein the centres of those primary, elongated elements, parts of which are within a distance of 1.5 or less, such as 1.2 or less, such as 1.1 or less times the radius of the first circle from the centre of the first circle, define the vertices of a first polygon with three or more sides.
The first polygon is a regular triangular polygon. At least part of the primary, elongated elements, in the cross section, define a triangular structure.
Preferably, none of the centres of the secondary, elongated elements, in the cross section, coincide with the centre of the first circle. The centres of at least part of the secondary, elongated elements, in the cross section, are positioned substantially along a line connecting the centres of two adjacent primary, elongated elements.
The optical fibre according to the present invention further comprises further, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the further, elongated elements having a refractive index being higher than a refractive index of any material adjacent to the further, elongated elements, and each having a centre not positioned outside the unit cell, and each having an area not exceeding the area of the unit cell.
At least part of the further, elongated elements, in the cross-section, define a triangular structure, a Honeycomb structure, or a Kagomxc3xa9 structure. Definitions for the Honeycomb and Kagomxc3xa9 structure is given above.
The further, elongated elements are at least partly comprised within the first circle. Preferably, the centres of at least part of the further, elongated elements substantially coincide with the centre of the first circle.
For a given unit cell, the sum of all areas of primary elements within the unit cell is larger than a constant times the area of that unit cell, said constant being larger than 0.1, such as larger than 0.15, such as larger than 0.2, such as larger than 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5, such as larger than 0.6, such as larger than 0.7, such as larger than 0.8.
In a third aspect the present invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:
a core region extending along the longitudinal direction,
a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising:
primary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the primary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
secondary, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the secondary elements having a refractive index being lower than a refractive index of any material adjacent to the primary elements,
xe2x80x83wherein any area of any primary element is larger than any area of any secondary element, and wherein, in a cross section perpendicular to the longitudinal direction
the primary, elongated elements define a triangular structure,
xe2x80x83wherein the periodic structure being, in the cross-section, defined by at least one unit cell, wherein, for each unit cell:
a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any primary, elongated elements, and wherein
the centres of any of the secondary, elongated elements, in the cross section, do not coincide with the centre of the first circle.
Any area of any primary element is larger than a constant times any area of any secondary element, said constant being larger than 1.1, such as larger than 1.2, such as larger than 1.3, such as larger than 1.4, such as larger than 1.5, such as larger than 2, such as larger than 5, such as larger than 10, such as larger than 20, such as larger than 50, such as larger than 100, such as larger than 200, such as larger than 500.
For each unit cell the sum of all areas of the secondary elements within the unit cell is larger than 0.005 times the area of that unit cell, such as larger than 0.01, such as larger than 0.05, such as larger than 0.1 such as larger than 0.15, such as larger than 0.2, such as larger th an 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5 times the area of that unit cell.
At least part of the secondary, elongated elements, in the cross-section, define a triangular structure, a Honeycomb structure, or a Kagomn structure. At least part of the secondary, elongated elements, in the cross section, have their centres positioned substantially along a line connecting the centres of two adjacent primary, elongated elements.
In the third aspect the fibre further comprises further, elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the further, elongated elements having a refractive index being higher than a refractive index of any material adjacent to the further, elongated elements, and each having a centre not positioned outside the unit cell, and each having an area not exceeding the area of the unit cell.
At least part of the further, elongated elements, in the cross-section, define a triangular structure, a Honeycomb structure, or a Kagomxc3xa9 structure.
The further, elongated elements, in the cross-section, are at least partly comprised within the first circle. Preferably, the centres of at least part of the further, elongated elements, in the cross section, substantially coincide with the centre of the first circle.
For a given unit cell, the sum of all areas of primary elements within the unit cell is larger than a constant times the area of that unit cell, said constant being larger than 0.1, such as larger than 0.15, such as larger than 0.2, such as larger than 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5, such as larger than 0.6, such as larger than 0.7, such as larger than 0.8.
According to the third aspect of the present invention the material adjacent to the elongated elements have a refractive index larger than 1.0, such as larger than 1.2, such as larger than 1.3, such as larger than 1.4, such as larger than 1.45, such as larger than 1.5, such as larger than 1.75, such as larger than 2.0, such as larger than 2.5, such as larger than 3.0, such as larger than 3.5, such as larger than 4.0.
The material adjacent to the elongated elements may comprise silica-based materials. Alternatively or additionally, the material adjacent to the elongated elements may comprise polymer-based materials.
Those elongated elements having a refractive index being lower than a refractive index of any material adjacent to the elongated element have a refractive index equal to 1. Preferably, those elongated elements having a refractive index being lower than a refractive index of any material adjacent to the elongated element comprise a vacuum, a liquid or a gas.
Those elongated elements having a refractive index being higher than a refractive index of any material adjacent to the elongated element have a refractive index larger than 1.3, such as larger than 1.4, such as larger than 1.45, such as larger than 1.5, such as larger than 1.75, such as larger than 2.0, such as larger than 2.5, such as larger than 3.0, such as larger than 3.5, such as larger than 4.0.
Those elongated elements having a refractive index being higher than a refractive index of any material adjacent to the elongated element comprise doped silica.
The aspects of present the invention relate to specific cladding structures and comprise no limitations what so ever on the core region.
In fact, the present invention should be taken as one relating to these specific cladding regions for use in any type of optical fibre in combination with one or more cores or core regions of any type.
Normally, in relation to periodic dielectric structures, the core is taken as an area of the structure, where the periodicity of the structure is broken. The photonic bandgap structure is designed so as to make light transmission impossible, and an altering of the periodicity will, consequently, make light transmission possiblexe2x80x94but only in the core and its close vicinity.
A number of different manners exist for defining the core. One manner is to replace one or more elements of the periodic structure with other elements with different refractive indices, cross sectional areas or shapes. Another manner is that the core has a periodic structure where only one or more elements are not present. Another manner is that the core also has a full periodic structure but this structure is different than the periodic structure of the cladding.
Preferably, the core region would comprise a first additional elongated element extending in the longitudinal direction of the fibre.
The core region may be defined as the smallest rectangular area comprising all elements breaking the symmetry of the at least substantially two-dimensionally periodic structure, the smallest rectangular area defining a first main axis and a second main axis, the first and second main axes having a first and a second length, respectively, the first length being equal to the second length.
Alternatively the core region may be defined as the smallest rectangular area comprising all elements breaking the symmetry of the at least substantially two-dimensionally periodic structure, the smallest rectangular area defining a first main axis and a second main axis, the first and second main axes having a first and a second length, respectively, the first length being larger than a constant times the second length, said constant being larger than 1.1, such as larger than 1.2, such as larger than 1.5, such as larger than 2, such as larger than 5, such as larger than 10, such as larger than 20, such as larger than 30, such as larger than 40, such as larger than 50.
An especially preferred first additional element is constituted by air, liquid or gas and being defined as a void in the material of the fibre, such as a void having a cross sectional area in the cross section being at least half the cross sectional area of the unit cell, such as at least one, such as at least 2, such as at least 3, such as at least 4, such as at least 5, such as at least 6, such as at least 7, such as at least 18, such as at least 36, such as at least 72 times the cross sectional area of the unit cell.
In that situation, the light may propagate almost entirely in a hollow core (e.g. containing a vacuum), which provides a number of advantages both for fibres used by the telecommunications industry as reduced propagation losses, improved dispersion properties and reduced non-linearities, and for fibres used in sensor applications, where e.g. a gas or liquid may be provided within the hollow core to obtain optimum overlap between the light and the gas or liquid.
In a number of different applications, the additional element or any material adjacent thereto may desirably comprise a dopant or a material showing higher order optical effects.
For communication purposes, higher order effects may be used for e.g. soliton communication.
For applications for fibre lasers or fibre amplifiers, the dopant may be e.g. a rare earth dopant adapted to receive pump radiation and amplify radiation travelling in the core region.
Alternatively, the dopant may be a light sensitive dopant, such as Germanium (e.g. loaded with other materials such as Ytterbium). In that situation, the dopant may be use for e.g. optically writing of a refractive-index grating in the fibre or core region.
For sensor applications, the dopant may be a material responsive to a characteristic of a gas or liquid, which response may be detected optically by light travelling in the core region.
In a number of applications, it is preferred that the core region comprises a second additional elongated element, the first and second additional elements being positioned at a distance where light travelling in one additional element is able to couple to the other additional element.
In one application, one elongated element may be a void holding a liquid or gas, which may be too turbid for light to travel through. In that situation, the light may travel in the other element while still coupling with the liquid or gas due to the limited distance between the elements.
In this situation, one may choose to have the liquid or gas travel only in one or both additional elementsxe2x80x94or even in all elongated voids, such as voids of the cladding structure.
Also, by providing two elements between which the light may couple, a number of optical devices may be provided, such as optical fibre couplers. The optical coupling between core elements or core regions may be designed so as to have a predetermined coupling at one or more defined wavelengths, which further makes a number of optical elements possible.
Another possibility is to include elongated electrical conductors in the fibre structure, such as ultra thin metal cylinders. Hereby, performances such as poling of the optical material may be realised. This could be relevant for a large range of materials, e.g., in silica or polymer structures. These kind of additional elements may eventually result in the realisation of actively controlled optical waveguide components such as switching elements.
As indicated above, specific advantages will be obtained also when the second additional element is a low-index cylinder.
In fact, due to the periodic structure of the present fibre, the fibre may easily be made to comprise a plurality of core regions.
These core regions may be provided sufficiently close for light travelling in one core region being able to couple to one or more core regions.
Alternatively, the core regions may be positioned spaced apart in order to provide a number of separate waveguides in a single fibre. In fact, the waveguides may be spaced sufficiently apart in order for their respective photonic bandgap structures to be different and e.g. be optimised for different wavelengths or wavelength regimes.
Preferably, the core regions are positioned symmetrically within the periodic structure, a period of the core regions being larger than a period of the periodic structure.
Naturally, a fibre of the present type may be used for a number of applications, where fibres are already used today.
In a fourth aspect, the invention relates to a sensor for sensing or detecting at least one characteristic of a liquid or gas, the sensor comprising:
a length of the optical fibre according to the invention, wherein the core region comprises at least a first additional element, the first element being a void extending along the longitudinal direction of the fibre,
means for providing the liquid or gas into the void of the core region,
means for introducing light into the core region, the light being adapted to interact with the gas or liquid in a manner so that the characteristic of the liquid or gas may be determined,
means for detecting light emitted from the fibre and for determining the characteristic of the liquid or gas.
At present, the characteristic may be absorption, absorbance, the presence of a specific agent or material in the gas or liquid, such as for use as a smoke detector, or any other characteristic sensed by an optical sensing method.
If the gas or liquid has a sufficiently low absorption at the wavelength of the light, the introducing means may be adapted to introduce the light into the first additional element. In that situation, an optimum overlap exists between the light and the liquid or gas.
Alternatively, the core region may comprise a second, elongated element extending in the longitudinal direction of the fibre, where the first and second additional elements are positioned at a distance where light travelling in one additional element is able to couple to the other additional element, and wherein the introducing means are adapted to introduce the light into the second additional element. In that situation, the sensing takes place via the light extending from the second to the first element.
In another type of sensor, the characteristic may not be sensed directly by light. In that situation, it may be desired to expose a suitable material to the characteristic, where the response of that material may be sensed optically, Thus, in this situation, at least part of an inner surface of the first additional element may comprise a layer of a material being adapted to alter in response to the characteristic of the gas or liquid, and wherein the introducing means is adapted to introduce light of a wavelength responsive to the altering of the material.
Naturally, the sensor may additionally comprise means for providing the gas or liquid in the fibre, such as for repeatedly providing gas or liquid therein, such as a gas pump if the sensor is used as a smoke detector.
In a fifth aspect, the invention relates to a fibre amplifier for amplifying an optical signal, said fibre amplifier comprising:
a length of optical fibre according to the invention, wherein the core region comprises a dopant material along at least part of the length, and
means for providing pump radiation to the dopant material for pumping the dopant material so as to amplify the optical signal.
Normally, fibre amplifiers will, further comprise means for spectrally separating the amplified optical signal from the pump signal, in order not to have pump radiation travelling in the fibre outside the amplifying region.
Especially for communication purposes, the dopant would comprise rare earth tons, such as erbium, ytterbium, praseodymium, neodymium, etc.
For other purposes, such as if it is desired to optically write gratings or other structures in the fibre or core region, or simply for modifying the refractive index of the core region, the dopant may comprise a photosensitive material, such as germanium, caesium, and/or photosensitivity enhancing co-dopants (e.g., hydrogen or deuterium).
In a seventh aspect, the invention relates to a fibre laser for generation of laser radiation, said fibre laser comprising:
a length of optical fibre according to any of the preceding claims, wherein the core region comprises a dopant material along at least part of the length,
means for providing pump radiation to the dopant material for pumping the dopant material so as to amplify the optical signal, and
feedback means for selectively feeding back at least part of the amplified optical signal so as to repeatedly pass the amplified optical signal through the length of the optical fibre so as to further amplify the optical signal.
Especially for communication purposes, the dopant comprises rare earth ions, such as Erbium, Ytterbium, Praseodymium, Neodymium, etc.
Also, the dopant may comprise a photosensitive material, such as germanium, in order to facilitate e.g. the writing of gratings in the fibre or core regionxe2x80x94or for increasing the refractive index of the core region.
The present invention also applies to PBG structures in the case of planar optical components fabricated using materials such as semiconductors and/or dielectric materials. The PBG effect may be obtained through the formation of parallel air filled voids in a silica-based planar waveguiding structure.
One example of such a component could be obtained using plasma enhanced chemical vapour deposition (PECVD) methods, where those skilled in the art know that it is possible (and sometimes difficult to avoid) to form air cylinders, when high ridges are overcladded. It is, therefore, according to the present invention suggested to refine these fabricational properties by opening up larger air cylinders, and to combine their possible appearances with the periodicity needed to define photonic crystal structures in which the optical power is guided along the cylinder axes. These properties may be used in a single plane, where a two-dimensional PBG structure may be defined, or in a further development in the fabrication of multi-level air-cylinders opening the possibility of forming three-dimensional structures according to the previously outlined designs.
Also, it may be desired to dope the fibre material, such as the material adjacent to the elongated elements. Alternatively, a layer of a material may be desired along the length thereof. In that situation, at least one of the preform elements may be coated or doped with a predetermined material.
In a eighth aspect, the present invention relates to a preform for manufacturing an optical fibre, the preform having a length in a longitudinal direction and a cross section perpendicular thereto, the preform comprising:
primary, elongated elements each having a centre axis extending in the longitudinal direction of the preform, the primary elements having a length in the longitudinal direction being essentially the same as the length of the preform,
inserted elements each extending in the longitudinal direction of the preform over a length being smaller than the length of the preform,
xe2x80x83the primary, elongated elements and the inserted elements form both a non-periodic structure and an at least substantially two-dimensionally periodic structure, the nonperiodic structure being surrounded by the substantially two-dimensionally periodic structure,
xe2x80x83the periodic structure being, in the cross-section perpendicular to the longitudinal direction, defined by at least one unit cell, wherein, for each unit cell:
a first circle is defined as the largest circular area possible having a centre not positioned outside the unit cell and not enclosing any part of any primary elements, the periphery of said first circle defining an inserted element.
The inserted elements, in at least part of the cross-section, defines a triangular structure. For each unit cell, the centres of those primary, elongated elements, parts of which are within a distance of 1.5 or less, such as 1.2 or less, such as 1.1 or less times the radius of the first circle from the centre of the first circle, define the vertices of a first polygon with three or more sides. The first polygon is a regular polygon.
Alternative, the first polygon has six or more sides, such as 12 or more, such as 18 or more, such as 36 or more.
The plurality of inserted elements are arranged along an axis extending in the longitudinal direction of the preform. At least part of the primary, elongated elements, in the cross-section, define a triangular structure, a Honeycomb structure, or a Kagomxc3xa9 structure. The outer surface of each of the primary, elongated elements may define a primary area, and the outer surface of each of the inserted elements may define a secondary area. The area of any primary area is different from any secondary area.
For each unit cell, the sum of all secondary areas is larger than 0.09 times the area of that unit cell, such as larger than 0.1, such as larger than 0.15, such as larger than 0.2, such as larger than 0.25, such as larger than 0.3, such as larger than 0.4, such as larger than 0.5, such as larger than 0.6, such as larger than 0.7, such as larger than 0.8 times the area of that unit cell.
In the preform any secondary area is larger than a constant times any primary area, said constant being larger than 1.1, such as larger than 1.2, such as larger than 1.3, such as larger than 1.4, such as larger than 1.5, such as larger than 2, such as larger than 4, such as larger than 7, such as larger than 10, such as larger than 20, such as larger than 50.
The primary, elongated elements may be hollow, whereas the inserted, elongated elements may be solid. The elongated elements may comprise silica-based materials. Alternatively, the elongated elements may comprise polymer-based materials.
The preform may further comprise further, elongated elements each having a centre axis extending in the longitudinal direction of the preform, and each having a centre not positioned outside the unit cell, and each having an area not exceeding the area of the unit cell, and each having a length in the longitudinal direction being essentially the same as the length of the preform, and each defining a further area being different from any area of primary, elongated elements. The further, elongated elements may be solid.
Regarding the position, the further, elongated elements are at least partly comprised within the first circle. Preferably, the centres of at least part of the further, elongated elements substantially coincide with the centre of the first circle.
The preform may further comprise a core region, the core region being defined as the non-periodic structure, the core region being surrounded by the at least substantially two-dimensionally periodic structure. Preferably, the core region comprises a hollow region, where the at least substantially two-dimensionally periodic structure surrounding the core region comprises at least two periods.
During the drawing process further initiatives may be employed to ensure large void filling fractions. This includes providing a gas in the voids of the fibre and sealing one end of the capillary tubes as disclosed in U.S. Pat. No. 5,802,236. Additionally for the present invention, it is preferred to seal the entire end of the preform by melting the jigs and the capillary (as well as any rods) together in one or both ends of the preform. Even sealing at specific locations along the preform may be of interest, if jigs are present at the specific locations. Alternatively, in a preferred embodiment only the voids not covering the high-index centres of the periodic cladding structure are sealed. Thereby the remainder of voids (those covering the high-index centres and therefore undesired) will collapse more rapidly than the voids which are desired to remain large.
Thus, in a ninth aspect, the present invention relates to a method for fabricating a preform, the preform having a length in a longitudinal direction and a cross section perpendicular thereto, the method comprising the steps of:
providing a holder for the preform, the holder having a predetermined shape and elongated grooves at its inner surface, the grooves having a length in the longitudinal direction being essentially the same as the length of the preform.
providing primary, elongated elements each having a centre axis extending in the longitudinal direction of the preform, the primary elements having a length in the longitudinal direction being essentially the same as the length of the preform,
providing secondary elements each extending in the longitudinal direction over a length being smaller than the length of the preform, and
positioning a plurality of secondary elements at essentially the same position along the longitudinal direction of the preform.
In a tenth aspect, the present invention relates to an optical fibre with a waveguide structure having a longitudinal direction, said optical fibre comprising:
a cladding region extending along the longitudinal direction, said cladding region comprising an at least substantially two-dimensionally periodic structure comprising elongated elements each having a centre axis extending in the longitudinal direction of the waveguide, the elongated elements having a refractive index being lower than a refractive index of any material adjacent to the elongated elements,
a core region extending along the longitudinal direction, said core region comprising at least one void extending along the longitudinal direction, a cross sectional area of said at least one void being larger than a constant times a cross sectional area of any elongated elements comprised within the cladding region, said constant being larger than 1.1, such as 1.3, such as 1.5, such as 1.7, such as 2, such as 3, such as 5, such as 10, such as 20, such as 50.
The centre of the rectangle may be defined as the centre of the smallest rectangular area possible, the centre being positioned not outside the core region, the rectangle enclosing the at least one void,
a rectangularity is defined as the length of the longest side of the rectangle divided by the length of the shortest side of the rectangle,
a first axis is defined as a longest vertice possible, the centre of the rectangle being positioned on said first axis, wherein each end of said first axis is enclosed within one of the at least one voids,
a second axis is defined substantially perpendicular to the first axis, the second axis being defined as a longest vertice possible, the centre of the rectangle being positioned on said second axis, wherein each end of said first axis is enclosed within one of the at least one voids, and
a eccentricity is being defined as the length of the first axis divided by the length of the second axis.
The rectangle may be a square, wherein the eccentricity is larger than one, such as 1.1, such as 1.3, such as 1.5, such as 1.7, such as 2, such 3, such as 5, such as 10. The rectangularity may be larger than one, such as 1.1, such as 1.3, such as 1.5, such as 1.7, such as 2, such 3, such as 5, such as 10.